This invention relates to lens mounts and specifically to a mount for an erector lens within a scope.
A rifle scope (or other gun scope) serves to magnify the target and overlay a visual aiming point, the reticle, on the target. It does this through the use of a series of lenses mounted within a generally tubular body and a combination of mechanisms to adjust the lenses.
The lenses in a scope can be broadly grouped into three categories: the objective lenses; erector lenses, and ocular (or eyepiece) lenses. Depending on the particular design there may be one or more individual lenses in each group within a scope. The objective lens gathers in the light from the target and prepares the image for the subsequent lenses, such as by magnifying the image, producing an inverted image of the target. The erector lenses further magnify or reduce the image and correct it to right side up. The ocular lenses coordinate the image and present it to the eye for viewing.
In a typical scope, the erector lenses are carried by a guide tube positioned within the scope body and mounted so that it pivots at one end. The opposite end of the guide tube can be adjusted horizontally and vertically to provide windage and elevation correction.
Variable magnification can be achieved by providing a means of adjusting the position of the erector lenses in relationship to each other within the guide tube. This is typically done through the use of a cam tube which fits closely around the guide tube. Each erector lens (or lens group) is mounted in an erector lens mount which slides within the guide tube. A guide sleeve attached to the erector lens mount slides in a straight slot in the body of the guide tube to maintain the orientation of the erector lens. This same guide sleeve also engages an angled, or curving, slot in the cam tube. Turning the cam tube causes the erector lens mount to move lengthwise within the guide tube, varying the magnification. Each erector lens will have its own slot in the cam tube and the configuration of these slots determines the amount and rate of magnification change as the cam. tube is turned.
In order for the scope to remain accurately sighted as the magnification is adjusted, it is critical that the alignment between the erector lenses and the guide tube remain constant throughout the movement. To achieve this, the tolerance between the outside diameter of the erector lens mount and the inside diameter of the must be very tight, typically on the order of 0.0004 in.
Unfortunately, the tubing from which the guide tube is manufacture is typically extruded and its inside diameter may vary by amounts in the range of 0.00025 in. This requires that conventional metal erector lens mounts must be made in several outside diameters. The guide tubes are then individually gauged and sorted into groups for use with the available erector lens mounts. This minimizes the fit problems but there are still inherent variations. In addition, the inside diameter of a guide tube may vary throughout the length of the tube within which the erector mount travels.
A loose fit between the erector lens mount and the guide tube may cause the mount to cock in the tube. This can change the aim point of the scope and can cause the mount (and thus the image) to jump when the mount catches and then frees up. Finer gradations in available erector lens mount diameters can improve the tolerances but at the cost of an increased number of parts in inventory and increased expense.
The manufacturing process for a conventional metal erector lens mount is a complex multi-step process. A typical process includes at least the steps of: lathe turning; drill and tap for the guide sleeve; grind to finish size; black oxide coat to reduce reflections; fit and secure the lenses; and clean to remove excess glue and contaminants. Some of these steps require the use of hazardous materials and/or may generate hazardous wastes. As a result, the erector lenses and mounts contribute significantly to the overall cost of a scope.
The same basic scope may be manufactured in several variations, or families, differing in features such as eye relief or field of view. These feature may require different erector lenses. Typically this means that different erector lens mounts and guide tubes will be used to accommodate the lenses. This further increases the number of parts which must be maintained in inventory and thus increases costs.
An effect known as coining also impacts the fit between the erector lens mount and the guide tube. When the slot for the guide sleeve is machined in the guide tube, it may release stresses within the tube and allow the tube to expand slightly. This alters the inside diameter of the tube and the size of the slot itself. This adversely effects the fit between the erector lens mount and the guide tube as well as the fit between the guide sleeve and the slot. Compressing the tube after machining can correct for these problems, but this is a relatively low precision operation and can introduce further problems. As such, it is typically not performed where conventional erector lens mounts are used.
Because both the erector lens mounts and the guide tube are typically metal, lubrication is required on the mating surfaces to provide a smooth sliding action. With time, this lubricant can migrate to the surface of the lenses degrading image quality. The lubricant may also increase the reflectance of the mating surfaces.
There is a need for an improved erector lens mount which is capable of maintaining tight tolerances when fitted to guide tubes having slight variations in inside diameter without requiring multiple erector lens mount sizes and without requiring gauging of the tubes to match the mounts. This will decrease the inventory parts count and associated cost. There is a need for an erector lens mount which will adapt to variations in inside diameter of the guide tube throughout its range of travel without adversely impacting image quality. There is a need for an erector lens mount which can accommodate sufficient variations to allow for the correction of coining in the guide tube. There is a need for an erector lens mount which is readily adaptable to different erector lenses for use with different families of scopes, thereby further decreasing parts inventory and cost. There is a need for such an erector lens mount which can be manufactured with a reduced number of steps and steps which reduce the use and production of hazardous materials or effluents from processing. There is a need for such an erector lens mount which operates without additional lubricant.
The present invention is directed to a synthetic erector lens mount for a rifle scope or similar. The mount is generally a tube with the lens mounted within the central passage and includes spring biasing fingers which push outward against the walls of the guide tube. Preferably the mount is injection molded from low reflectance, low friction, or self lubricating, plastic.
According to the invention there are provided glides, or rails, which ride on the inner surface of the guide tube and are arranged generally opposite the fingers so that the fingers urge the glides into contact with the guide tube.
According to an aspect of the invention the fingers may include a raised portion intended to contact the guide tube, may be angled outward toward the tube, or both.
According to another aspect of the invention the glides have a gap spanning the midpoint of the mount so that an imperfection in the wall of the guide tube will not cause the mount to rock at the midpoint.
Further in accordance with the invention the glides have an outermost point with a radial distance from the centerline of the mount which is substantially equal to the nominal radius of the guide tube.
Still further in accordance with the invention, the mount has a flat recess in its inner wall for seating a nut and an aligned and parallel flat area in the outer wall so that a guide sleeve can be seated on the outer flat area and a screw passed through the sleeve and wall of the mount and threaded into the nut so that the nut and sleeve sandwich the wall of the mount between them, providing a stable mount for the sleeve.
The advantages of such an apparatus are a lens mount which is easily manufactured and adapts to slight irregularities in the inner wall of a guide tube and to variations in the inner diameter from one tube to another. This allows the mount to maintain tight fit tolerances without requiring multiple sizes of mounts to be matched with tubes having inner diameters varying from nominal. This adaptation will also accommodate variations due to either coining or the correction of coining. By interchanging the inner core of the injection mold, the mount can be adapted to multiple lenses while maintaining the same outside profile, allowing the use of a single size of guide tube for the multiple lenses. The manufacture of the mount eliminates many of the conventional manufacturing steps because required features can be molded into the mount. Proper selection of the plastic can provide a low glare, low friction material eliminating the need to plate and lubricate the mount.
The above and other features and advantages of the present invention will become more clear from the detailed description of a specific illustrative embodiment thereof, presented below in conjunction with the accompanying drawings.